Elastic potential energy is the recoverable work under the unloading curve

Steps:

• Loading curve OY shows total deformation (elastic + plastic) up to point Y.
• Unloading curve YZ retraces elastically, with residual plastic extension at Z.
• Areas: P likely hysteresis loop (energy lost as heat); Q plastic deformation zone; R area under YZ (recovered elastic energy).
• Maximum elastic PE stored equals work recoverable on unloading, represented by area R.

Why D is correct:

• Elastic potential energy $U = \frac{1}{2} F \Delta x$ for linear elastic region, but here it's the integral under unloading curve YZ, matching area R per work-energy principle.

Why the others are wrong:

• A. P is hysteresis area, representing dissipated energy, not stored elastic PE.
• B. Q is plastic deformation zone, indicating permanent strain with no recoverable energy.
• C. O+R includes origin to start plus R, but adds extraneous unloaded region irrelevant to stored energy.

Final answer: D